Australia's conservative national government, which opposes homosexual marriages, said on Thursday it will overturn any new law legalizing gay civil unions in the national capital.

"There is a special place in Australian society for the institution of marriage, as historically understood, and we do not intend to allow that to be in any way undermined," said Prime Minister John Howard.

The Australian Capital Territory (ACT) government, which rules the capital, Canberra, has become the first state or territory to introduce same-sex civil union legislation.

The ACT hopes to pass the legislation into law by May, but the federal government has constitutional control over the nation's two territories, though not its six states, enabling it to overturn laws.

Attorney General Philip Ruddock said the Commonwealth (federal) government would veto any law which elevated gay civil unions to the status of marriage.

"Let me make it very clear, that will not satisfy the Commonwealth and we would include the introduction of legislation to prevent that from occurring," Ruddock told reporters.

"If they seek to portray civil unions as a marriage, in our view, that is quite inappropriate. It is quite misleading, it suggests to people who might be interested in civil union that what they have is a marriage, when in fact it is not," he said.

The ACT's civil union will only give gay couples equality with married couples regarding wills and the division of property in the territory. While the civil union is open to all Australians it is only valid in the ACT and will not affect national laws governing taxation, superannuation and health care.

ACT chief minister Jon Stanhope said on Thursday he would push ahead with the legislation and questioned why the national government opposed gay unions.

"What is his (Ruddock's) real concern about my commitment to remove discrimination and to show respect to same-sex relationships," asked Stanhope.

"One has to pose the question whether or not the real reason is that there is no place in John Howard's Australia for homosexuals."

Britain introduced gay civil unions in December 2005, with singer Elton John and his partner David Furnish among hundreds of gay couples who tied the knot.

A number of other countries, including Canada, Spain, France, Argentina, Denmark, Norway, Sweden and the Netherlands, allow for formal recognition of same-sex relationships.

Australia's conservative federal government threatened to block legislation submitted to local parliament in the country's capital territory that would recognise civil unions for gay and lesbian couples.

The center-left government that runs the Australian Capital Territory (ACT) under Australia's state system introduced the bill in the local legislature earlier this week.

The legislation would be the first in Australia to allow same-sex couples to enter into a civil union that would have the same recognition as marriage.

The bill was introduced in defiance of a federal law passed two years ago by the government of Prime Minister John Howard that formally defined marriage as a union between a man and a woman.

Federal Attorney-General Philip Ruddock said Thursday that the national law gave him the authority to block the ACT bid to legalise gay unions.

"For a territory to say 'Well, that's fine for the Commonwealth parliament to have resolved in that way, we're still going to assert that a civil union is a marriage in all but title, and we're going to use marriage celebrants to demonstrate that', let me make it very clear: that will not satisfy the Commonwealth," Ruddock said on Australian Broadcasting Corporation radio.

The head of the ACT government, John Stanhope, said Ruddock's reaction revealed the homophobia of the Howard administration.

"The question is whether or not the real reason (for Ruddock's position) is that there is no place in John Howard's Australia for homosexuals," he said.

But Stanhope said the government's position could force him to amend his bill, which is due to be debated in the ACT legislature in May.